Is there a way to add motion blur to a text object animated via the animation-nodes addon?
What you are asking for seams more like ghosts of previous frames/numbers or slower fading of those, rather than motion blur.

It can be done several ways, depending on the outcome and render, without resorting to actual motion blur and sampling. Considering that the motion blur would slow renders and the rest of the scene may not need it.

Steps for the Animation Nodes part are pretty much the same:


search for time generator (a node that formats as 00:00:00..)
have several instances of the text object (that "display" numbers)
It has to have deep copy to have different data/texts
have a delay for the frame for each instance. In a loop, using instance
assign a convenient property to respective instances to further use for transparency, variation in color or other means of "fading them"


after this you can set and use respective property depending on the convenient render or situation:


use pass_index (object index) to read in cycles or compositor
(it's only integers, so may wanna use 100 or 100/instances or so to get a 0-1 factor in the end)
use object color, directly, if you use blender internal. That is just color
set a different layer and use compositor render layers


If it's just a screen timer, I would choose a compositor version with all the text and ghosts in separate render layers, that you can further tweak and glow etc. For that I would use alpha and not a serious shading and just adjust in compositor as color etc, see below.



Let's see the animation nodes setup for this:

The basis, create instances and assigns delayed times to them.



Setting the convenient parameter to vary:


Set object index (pass index) as 20, 40 etc instead of 0.2, 0.4 ..
(needed if cycles, or compositor)
Setting the layer, just use object/layers node
(The compositor can use layers or object index)
Maybe a transform object out to mave the ghosts under the main text a bit
Setting the object color for blender internal if the case




Example of using the index in cycles

Cycles only allows vertex color (mesh only :( ) or object index





For compositor the way I would use it:

(same works very well without compositor, you just don't get blur)

I would use layers/render layers and a flat emission for all, then compositor mixing, like this:



note that:


If you use this compositor/cycles/An many windows all along, may wanna uncheck AN panel left auto exec and check only the 3 under. Otherwise it may update the compositor continuously.
+
I add 1 to the index
I use Z transform qith a small 002 (works great for simple cycles too)
I use 2 layers / 2 renderlayers and render is set to transparent
I use the alpha in compositor, + bit of blur for ghosts 
(may do extra styling for compositor..)